In PCT/EP2009/051702, which is the closest prior art, an eccentric ring is interposed between the crankpin and the big end bearing of the connecting rod. The eccentric ring rotates in synchronization to the crankshaft by means of a set of gear wheels. The rotation of a control member changes the phase between the crankshaft and the eccentric ring and so controls the compression ratio.
The necessary eccentricity between the inner and outer cylindrical surfaces of the eccentric ring is small, for instance a 4 mm eccentricity enables a compression ratio range between 8:1 and 18:1 in an engine having 100 mm piston stroke.
Among the drawbacks of the closest prior art are the degradation of the crankshaft strength, the increased complication, size and cost, the additional inertia vibrations, the need for special cylinder arrangement in order to keep reasonable the number of additional parts.